Amosulalol hydrochloride is an excellent hypotensive agent having an adrenergic .alpha.-blocking action and an adrenergic .beta.-blocking action. In general, it is desirable that the frequency of administering a hypotensive agent be minimized from the simplicity of clinical therapy but considering the biological half life of amosulalol hydrochloride, it is difficult to realize the above-described goal by conventional formulations.